# How is radiometric dating used to determine the age of rocks?

### 2 Answers

- ZirpLv 71 month ago
Some chemical elements have several isotopes. Some of those are unstable and fall apart, releasing radiation.

We've figured out how fast every kind of isotope falls apart. they all have a "halflife", the time in which half the atoms of an isotope decay and in which the intensity of the radiation is halved.

the most famous one is carbon-14, which has a half-life of about 5730 years. If a piece of rock contains a quarter of the amount of carbon-14 it had in the beginning, we know two halflives have past so the sample must be close to 11460 years old.

Obviously, isotopes with a halflife of (milli)seconds or millions of years aren't practical

- CRRLv 71 month ago
If there is a radioactive element we can use the decay rate and amount of daughter products to give an estimate of the age of the rock; provided we assume that we know the original proportions and the rock is a closed system. In reality rocks produced by observed volcanic eruptions produce ages produce ages that don't match and can range over orders of magnitude. Samples from the same geological layer can produce a wide range of results from different methods and different sub samples.

In practice you can often find at least one radiometric date that is close enough to be adopted to confirm what you already believe.